Monogramming attachment for sewing machines

ABSTRACT

A work shifting attachment for a zigzag sewing machine in which an exchangeable pattern cam disk with a cam track formed in each face may be inserted through a hatch in the attachment cover into engagement with a cam drive and into cooperative relation with a pair of cam followers connected to shift a work clamp in a path appropriate for monogramming.

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[54] MONOGRAMMING ATTACHMENT FOR [56] References Cited SEWHNG MACHINES UNITED STATES PATENTS 1 19mph Rams 2,907,290 10/1959 Burgess et al. ..112/77 x Green Island- Toms Rlver NJ. 08753 3,033,136 5/1962 Nickerson et al... ..112/77 s p EROS [2|] Appl. No.: 71,380 Primary ExaminerH. Hampton Hunter Mummy-Marshall J. Breen, Chester A. Williams, Jr. and Related 05. Application Data Robert s i [63] Continuation-in-part of Ser. No. 55,330, July 16,

1970, abandoned. I [57] ABSTRACT A work shifting attachment for a zigzag sewing machine in [52] U.S.Cl ..112/77 which an exchangeable pattern cam disk with a cam track [51] Int. Cl. ..D05b 3/24 formed in each face may be inserted through a hatch in the at- 53 tachment cover into engagement with a cam drive and into Field of Search ..112/77, 102, 158 A, 70

cooperative relation with a pair of cam followers connected to shift a work clamp in a path appropriate for monogramming.

6 Claims, 9 Drawing Figures PATENTEnmzeme 3.651.773

saw 2 OF 3 as N 50 I0! 24 IN VENTOR.

Joseph G. Rakocs TTORNEY WITNESS: mm W PATENTEUMAR28 1912 3,651 J73 SHEET 3 OF 3 Fig. 9

INVENTOR.

Joseph G.-Rc|kc|cs WITNESS: mu

fl WM ATTORNEY MONOGRAMMING ATTACHMENT FOR SEWING MACHINES CROSS REFERENCE TO RELATED APPLICATIONS This is a eontinuation-in-part of my application Ser. No. 55,330 filed July 16, 1970, now abandoned.

BACKGROUND OF THE INVENTION Devices which have been utilized heretofore to effect decorative stitch groups, such as monograms, have included on the one hand work shifting mechanisms built into the machine frame such as those shown in U.S. Pat. No. 268,804, Dec. I2, 1882 or U.S. Pat. No. 3,112,718, Dec. 3, 1963, or on the other hand work shifting mechanisms which are attachments to a sewing machine such as those shown in U.S. Pat. No. 1,575,285, Mar. 2, 1926 or U.S. Pat. No. 3,360,508 ofFeb. 20, 1968.

Those known devices which are built into the machine frame take up so much room in the machine casing as to permanently commit the machine to the special purpose of monogramming. The known attachments for monogramming have required such large proportions as to seriously limit the practicality of their use and further more to pose serious risk of physical damage to the sewing machine to which they are applied.

SUMMARY OF THE INVENTION It is an object of this invention to provide a work shifting monogramming attachment for a sewing machine which is no larger and presents no greater cantilever effect than conventional buttonhole attachments, and therefore, does not physically threaten the sewing machine structure to which it is clamped.

Another object of this invention is to provide a monogramming attachment of the above character, a means for conveniently exchanging pattern cam information to produce any selected monogram.

The aforegoing objects and advantages of this invention are attained by providing a monogramming attachment for a sewing machine which utilizes as an information carrier a rotating cam disk of which opposite faces are each formed with cam track so that the maximum information can be arranged on a cam disk of given diameter. Sufficient work shifting information to produce any one letter of the alphabet can be stored on a cam disk of sufficiently small diameter as to be housed completely within an attachment cover no larger than that ordinarily utilized for conventional buttonhole attachments.

Convenient exchange of pattern cam disks in the attachment cover is provided for by means of a novel hinged construction for the follower mechanism of the uppermost cam track in the pattern cam disk for the attachment.

DESCRIPTION OF THE DRAWINGS With reference to the accompanying drawings the description of a preferred embodiment is illustrated in which:

FIG. 1 is a top plan view of a sewing machine work shifting attachment having this invention applied thereto, with the attachment cover and the pattern cam removed,

FIG. 2 is a left-side elevational view of the attachment of FIG. I with the pattern cam in place and showing the attachment cover in vertical cross section,

FIG. 3 is a right-side elevational view of the attachment similar to FIG. 2,

FIG. 4 is a top plan view of a pattern cam usable with the attachment of FIG. 1,

FIG. 5 is a bottom plan view of a pattern cam usable with the attachment of FIG. 1,

FIG. 6 is a top plan view of the attachment of FIG. I with the pattern cam follower mechanism removed and with the feed blade broken away to expose the pattern cam drawing ratchet mechanism,

FIG. 7 is a perspective view of the attachment of this invention with the cover lid opened and the upper cam follower lever swung up into a position for exchange of pattern cams,

FIG. 8 is a enlarged perspective view of the ratchet wheel and the pattern cam seat secured thereto, and

FIG. 9 represents an exploded top plan view of the attachment frame and the operating parts of the attachment.

Referring particularly to FIG. 7 of the drawings, in which the complete monogramming attachment of this invention is illustrated in perspective, 1] indicates an attachment cover which may be fonned of stamped sheet metal, plastic, or the like and shrouds most of the operating mechanism of the attachment. An access opening 12 formed in the top of the cover is closed by a lid 13 secured by a hinge 14 at the rear of the cover. A leaf spring 15 is carried on the underside of the lid for hearing downwardly upon mechanism on the attachment when the lid is closed, and a rotatable locking lug l6 journaled near the front edge of the lid is connected, as shown in FIGS. 2 and 3 with a finger grip l7 exteriorly of the lid so that the lug may be turned into locking relation beneath the cover when the lid is closed.

AS shown in FIG. 7, only a limited number of the operating parts of the attachment project exteriorly of the cover. At the front of the attachment these exposed parts include a portion of the fabric shifting feed blade 20, a work engaging foot 21 pivoted to the feed blade and formed with an opening 22 through which a monogram may be stitched, and a portion of a stripper foot 23 for preventing flagging of the work in the opening 22. Also projecting out of the attachment frame at the front is a portion of the attachment frame 24 which is formed with a sewing machine presser bar accommodating seat 25 by which the attachment may be secured in place on a sewing machine in a manner which is conventional for buttonhole attachments. Adjacent to the presser bar accommodating seat 25, an operating lever arm 26 projects out of the attachment cover and is formed with a bifurcated extremity 27 adapted as is conventional, to embrace a needle clamp on a sewing machine needle bar to drive the attachment. As shown in FIG. 7, a manual indexing lever 28 projects laterally out of the attachment cover, and at the opposite side of the attachment, not visible in FIG. 7 but as shown in FIG. 6, a stitch density ad justing tab 29 and a locking nut 30 for securing the tab 29 in selected position protrude from the cover 1 I.

All of the remaining mechanism of the attachment of this invention except the few portions described above are contained within the cover 11. FIG. 7 illustrates the lid 13 fully opened exposing a pattern cam 40 in operative position in the attachment. A follower lever 41 which carries a follower pin 42 for tracking a cam groove 43 in the top surface 44 of the pattern cam is shown in FIG. 7 in a substantially vertical thrown-out position which completely clears the area vertically above the pattern cam 40 so that the cam may be exchanged readily.

As shown in FIGS. 4 and 5, the pattern cam 40, which takes the form of a circular disk with a serrated periphery 45 is formed with a central opening 46 which is formed with a central opening 46 which is non-circular. In addition to the cam groove 43 in the top surface 44, the pattern is formed with a different cam groove 47 in the bottom surface 48. The bottom surface 48 is distinguished by an axially projecting boss 49 about the central opening 46 which prevents the pattern cam from cooperating with the followers if the cam is placed in the attachment wrong side up.

Referring to FIGS. 1, 2, 3, 6, 8 and 9, the attachment frame 24 comprises a generally flat plate 50 formed at the front with the previously mentioned presser bar accommodating seat 25. Rearwardly of the presser bar accommodating seat, the frame is formed with laterally spaced bearing standards 51 which are transversely bored as shown at 52 in FIG. 8. Rearwardly of the standards, the frame is formed with a vertical aperture 53 accommodating a shouldered pivot pin 54 for the feed blade 20. A beehive spring 55 on the pivot pin 54 held on by a spring clip 56 provides for resilience in the feed blade support in the event of uneven thickness of a work fabric. A clearance opening 57 rearwardly adjacent to the pivot pin 54 in the frame accommodates a narrow offset portion 58 of the feed blade 20.

The feed blade includes a lower substantially horizontal portion 59 situated beneath the attachment frame and to which the work engaging foot 21 is pivoted. The lower portion 59 is formed with a slot 60 which embraces the pivot pin 54. The feed blade is formed with an upper substantially horizontal portion 61 which extends rearwardly from the offset portion 58 and rests upon the tops of two posts 62 of intermediate height projecting upwardly from the plate 50 of the frame one post on each side of the clearance opening 57. As shown in FIG. 9, the upper horizontal portion 61 of the feed blade 20 is formed with a clearance opening 63 and with mutually perpendicular slots 64 and 65 one slot 64 being arranged substantially lengthwise of the feed blade and the other slot 65 extending transversely thereof.

Referring to FIGS. 6 and 9, a cam driving mechanism is carried by the attachment frame and arranged largely beneath the upper horizontal portion 61 of the feed blade. As shown in FIG. 9, a bearing stud 69 is secured vertically in the flat plate portion 50 of the attachment frame. A washer 70 which secured as by a force fit on the stud, has a large diameter shoulder 71 formed concentrically on the underside, while on the upperside is formed two successively smaller diameter concentric shoulders 72 and 73 journaled beneath the washer 70 on the shoulder 71 is a manual cam indexing lever 74 having one arm 75 to which the previously mentioned manual lever arm 28 is secured. Another arm 76 of the cam indexing lever 74 carries a pivoted ratchet pawl 77 which is biased by a spring 78 inwardly toward the stud 69. A coil spring 79 stretched between the manual lever arm 28 and a reinforcing rib 80 on the attachment frame biases the cam indexing lever counterclockwise as viewed in FIGS. 6 and 9.

Arranged directly above the washer 70 and journaled on the first upper shoulder 72 is a pawl carrying lever to which is pivoted a ratchet pawl 91 biased upwardly toward the stub shaft 69 by a spring 92. The pawl carrying lever 90 is oscillated in response to sewing machine operation by a linkage connection with the sewing machine needle bar oscillated lever arm 26 described earlier. To this end, and as shown in FIG. 3, the lever arm 26 comprises one arm of a bell crank secured to a fulcrum stud 93 which is journaled in the transverse bores 52 in the frame bearing standards 51 and held therein by a retaining clip 94. A bell crank arm 95 depending from the lever arm 26 has pivoted thereto an angle bracket 96 which is drilled and tapped to accommodate a clamp screw 97. A link 98 pivoted at 99 to the pawl carrying lever 90 is formed with a slot 100 accommodating the clamp screw 97 with capacity for united adjustment, and a finger tab 101 extending from the link 98 facilitates setting of this limited adjustment so that the attachment may be readily adapted to sewing machines with needle bar strokes of differing elevations.

Journaled on the smallest diameter shoulder 73 above the washer 70 is a stitch density control lever from which an operating arm 111 extends and from which arm the previously mentioned adjusting tab 29 extends. A stop projection 112 extending radially from the stitch density control lever 110 engages the pawl arm 76 of the cam indexing lever and a radially enlarged pawl throwout section 113 is formed on the density control lever 110 for cooperation with the driving pawl 91. A link 1114 which is pivoted at 115 to the operating arm 111 of the stitch density control lever is slotted as at 116 (FIG. 2) to accommodate a threaded stud 117 secured to the attachment frame and the lock nut 30 is threaded on to the stud 117 to clamp the stitch density control lever in selected position.

Joumaled on the stud shaft 69 above the stitch density control lever 110 is a ratchet wheel 120 formed with a wide circular flange 121 which in the periphery is formed with ratchet teeth 122 which do not extend completely about the flange but which terminate at each side of a smooth peripheral portion 123 which preferably is of an angular extent at least equal to the maximum oscillation of the drive pawl 91. This interrupted ratchet wheel will serve to prevent automatic repetition of a cycle of pattern cam tracking in this attachment so that the information on the cam tracks need not be continuous where this interruption occurs. Removal of this limitation from the pattern cam design permits an appreciable increase in the usable information which may be included on the pattern cams as will be described further hereinbelow.

As shown in FIG. 8, a central spindle 124 is formed above the flange 121 of the ratchet wheel and secured so as to turn therewith is a pattern cam seat element 125. Preferably, the seat element 125 includes a flange 126 for axially supporting a pattern cam which is placed thereon, and a non-circular hub 127 shaped to coincide with the non-circular shape of the central opening 46 in the pattern cams 40. The non-circular shape be provided by a shafted portion 128 of an otherwise circular hub.

A leaf spring 130 secured by screws 131 to the flat frame plate 50 overlies and frictionally resists turning movement of the ratchet wheel 120. The pattern cam seat element 125 however, projects upwardly through the clearance opening 63 in the upper horizontal part 61 of the feed blade 20.

Referring to FIG. 9, the flat attachment frame plate 50 is formed with three supporting posts which terminate in substantially co-planar relation and at a higher elevation than the feed blade supporting posts 62. Each of the supporting posts 140 is formed with a vertically drilled and tapped hole 141 to accommodate fastening screws 142 which secure a pattern cam follower supporting plate 143 to the posts 140.

Pivoted at 144 near the rear and at one side of the plate 143 is a follower lever 145 which carries adjacent to the free extremity a transverse pin 146 extending above the follower lever and a connection pin 147 extending below the follower lever. Below the follower lever, the connection pin 147 extends through the lengthwise slot 64 in the feed blade 20; while above the follower lever, the pin 146 serves to track the bottom cam groove 47 in a pattern cam which is placed on the cam seat 125. The cam groove 47 and the follower lever 145, therefore, serve to impart angular movement to the feed blade 20 about the pivot pin 54.

A slide plate 150 formed with offset parallel guide slots 151 and 152 is constrained on the follower supporting plate 143 by shouldered fastenings 154 and 155 respectively, which fastenings may take the form either of shouldered rivets or shouldered screws. The guide slots provide for limited movement of the slide plate lengthwise of the monogramming attachment and a pin 155 depending from the slide plate and passing through a clearance slot 156 in the follower supporting plate 143 extends through the transverse slot 65 in the feed blade 20.

The slide plate 150 is formed with laterally spaced upstanding ears 157 between which extends a pivot pin 158 that is also embraced by downtumed ears 159 formed on the follower lever 41 previously described. The follower pin 42 on the follower lever 41 tracks the upper cam groove 43 in a pattern cam on the cam seat 125 when the follower lever 41 is swung down. The top cam groove 43, therefore, controls the lengthwise sliding movement of the feed blade.

The pivoted pattern cam follower lever 41 is formed with a stabilizing slot 170, which as shown in FIG. 1, embraces the top of the stud 69 when the follower lever is lowered into cam tracking relation. The follower lever 41 is also formed near the free extremity with an aperture through which the top of a pattern cam will be visible when a cam is placed on the cam seat 125 and the lever 41 is turned down into cam tracking relation. The inner wall of the aperture 175 is formed with a blunt arrowhead 176 and similarly the outer wall of the stabilizing slot is also formed with a blunt arrowhead 177 one or both of which cooperates with an indicium 178 in the form of a radial line on the top of the pattern cam to show the pattern cam position appropriate to the start of a monogram. The shape of the monogram which will be produced by each pattern cam may also be marked as at 179 on the top surface of the cam.

The attachment of this invention is applied to a sewing machine of the zigzag type in a manner which is conventional for buttonhole making attachments, that is, the feed mechanism of the sewing machine is disabled either by dropping the feed or by raising or covering the sewing machine throat plate and the seat 25 on the attachment frame is clamped to the presser bar of the sewing machine with the bifurcated operating lever arm 26 embracing the needle clamp on the sewing machine needle bar. With the sewing machine controls set to provide for a zigzag motion to the needle bar, the attachment needs only insertion of a desired pattern cam in order to be in readiness to produce a monogram. The cover lid 13 and the follower lever 411 are opened as shown in FIG. 7 in order to make the pattern cam 40 accessible for exchange. When the desired replacement cam 40 is positioned on the cam seat 125 with the pin 146 of the follower lever 145 tracking the bottom groove 47 of the cam, the pattern cam will be in proper position. The follower lever 41 is then lowered and the work engaging foot shifted slightly until the follower pin 42 engages with the top cam back 43. With the lid 13 remaining open, the operator then manually indexes the pattern cam around by repeatedly oscillating the manual lever arm 28 until the indicium 1178 comes into alignment with one of the arrowheads 176 or 177. It will be appreciated that the shape of the cam grooves will differ as between the pattern cams for a font and a clear space for the indicium 178 will exist at varying distances from the center of the pattern cams, hence, the advantage of two arrowheads 176, 177.

The lid 13 may be closed when the starting position of the monogram has been set. The density of stitching as reflected by the number of ratchet teeth 122 which the driving pawl will traverse on each oscillation of the operating lever 26 can now be adjusted by loosening the clamp nut 30 and shifting the tab 29. This will position the throw out section 113 more or less in the path of the driving pawl 91 and therefore will regulate the effective angular movement of the pawl while in engagement with the ratchet teeth 122.

The attachment and sewing machine after the above described adjustments will be ready to produce a monogram with a work fabric placed beneath the work engaging foot 21, the sewing machine may be operated to produce the selected monogram within the opening 22 of the work engaging foot.

The pattern of stitches for most monograms is not closed loop. For instance, with the letter S for which the cam is illustrated in FIGS. 41 and 5, the beginning and ending of the letter do not coincide. The transition, therefore, from the completion of the letter back to the starting point is not intended to be used for a sewing operation. The plain portion 123 of the ratchet wheel periphery along which the ratchet teeth are interrupted, serves to terminate the work shifting operation of the attachment. This plain portion of the ratchet wheel is oriented with respect to the pattern cam seat 125 to come into cooperation with the pawl 91 when the monogram has been completed. When the attachment ceases to shift the work, and several tying stitches in substantially the same location have been made, the operator will know that the monogramming operation is complete and the work can be removed.

A second monogram can then be made very simply by replacing fresh work under the work engaging foot and just slightly shifting the pattern cam by use of the manual lever arm 23 to bring the operating pawl 91 into engagement with the ratchet teeth 122 adjacent the plain portion 123 of the ratchet wheel.

Having thus set forth the nature of this invention, what is claimed herein is:

1. A sewing machine work shifting attachment for monogramming comprising an attachment frame adapted to be secured to a sewing machine and including a cover formed with an opening, a work engaging foot shiftably supported on said frame, means for shifting said work engaging foot in a predetermined path includin two separate cam follower linkages effective each to contro the movement of said work engaging foot in mutually perpendicular direction, a cam seat in said attachment frame for an exchangeable pattern cam disk insertable axially through said opening of said attachment frame cover, said cam follower linkages being arranged to track cam surfaces in opposite sides of a pattern cam on said cam seat, said cam follower linkage which is arranged to track a cam surface in that side of a pattern cam which faces said attachment frame cover opening including an articulated cam follower portion which may be swung outwardly through said opening in the attachment frame cover to facilitate exchange of pattern cam disks.

2. A sewing machine work shifting attachment for monogramming comprising an attachment frame adapted to be secured to a sewing machine, a cam shaft joumaled on said attachment frame and having a free extremity exchangeably to accommodate thereon pattern cam disks formed with a cam track in each side, mechanism operable by said sewing machine for imparting step-by-step turning movement to said cam shaft, a work shifting lever slidably pivoted on said attachment frame and formed with a feed blade extending substantially perpendicular to said cam shaft, said feed blade being formed with a clearance aperture embracing the free extremity of said cam shaft and being formed adjacent to said clearance opening with a pair of mutually perpendicular drive slots, a cam follower supporting platform secured to said attachment frame substantially parallel to said feed blade, a pair of cam follower members each carrying a connection element extending through a respective one of said feed blade drive slots, means shiftably constraining said cam follower members on said platform for movement of said connection elements in substantially mutually perpendicular directions, a first of said cam follower members carrying a cam follower element extending in the same direction as said cam shaft free extremity for engagement with a cam track in the under face of a pattern cam on said cam shaft, a second of said cam follower members being formed with an articulated portion which may be swung alternatively to positions parallel to the cam shaft for exchange of pattern cams or perpendicular to the cam shaft for operation of the attachment, and a cam follower element carried by said articulated portion for engagement with a cam track in the exposed face of a pattern cam on the cam shaft.

3. A sewing machine work shifting attachment for monogramming as set forth in claim 2 in which said second of said cam follower members includes a plate carrying said connection element, cooperating means on said plate and on said supporting platform for constraining said plate to partake only of translatory movement, and a pair of ears struck up from said plate, in which said the articulated portion of said second cam follower includes a lever arm formed with a pair of downturned ears overlapping the ears of the plate, and in which a pivot pin extends between the overlapped ears of said plate and lever arm.

1. A sewing machine work shifting attachment for monogramming as set forth in claim 3 in which said lever arm is formed with an aperture snuggly accommodating the free extremity of the cam shaft.

5. A sewing machine work shifting attachment for monogramming as set forth in claim 4 in which said lever arm is formed with a viewing aperture through which a pattern cam in place on the cam shaft is visible accurately to indicate the position of rotation of a pattern cam.

6. A sewing machine work shifting attachment for monogramming as set forth in claim 2 in which the mechanism operable by said sewing machine for imparting step-by-step turning movement to said cam shaft includes a toothed ratchet wheel fast with respect to the cam shaft, a pawl oscillated by said sewing machine relatively to said ratchet wheel, said ratchet wheel being formed with a segment without teeth at least as large angularly as the angular oscillation of said pawl, and in which manually operable meansare provided for at-will turning of said cam shaft. 

1. A sewing machine work shifting attachment for monogramming comprising an attachment frame adapted to be secured to a sewing machine and including a cover formed with an opening, a work engaging foot shiftably supported on said frame, means for shifting said work engaging foot in a predetermined path including two separate cam follower linkages effective each to control the movements of said work engaging foot in mutually perpendicular direction, a cam seat in said attachment frame for an exchangeable pattern cam disk insertable axially through said opening of said attachment frame cover, said cam follower linkages being arranged to track cam surfaces in opposite sides of a pattern cam on said cam seat, said cam follower linkage which is arranged to track a cam surface in that side of a pattern cam which faces said attachment frame cover opening including an articulated cam follower portion which may be swung outwardly through said opening in the attachment frame cover to facilitate exchange of pattern cam disks.
 2. A sewing machine work shifting attachment for monogramming comprising an attachment frame adapted to be secured to a sewing machine, a cam shaft journaled on said attachment frame and having a free extremity exchangeably to accommodate thereon pattern cam disks formed with a cam track in each side, mechanism operable by said sewing machine for imparting step-by-step turning movement to said cam shaft, a work shifting lever slidably pivoted on said attachment frame and formed with a feed blade extending substantially perpendicular to said cam shaft, said feed blade being formed with a clearance aperture embracing the free extremity of said cam shaft and being formed adjacent to said clearance opening with a pair of mutually perpendicular drive slots, a cam follower supporting platform secured to said attachment frame substantially parallel to said feed blade, a pair of cam follower members each carrying a connection element extending through a respective one of said feed blade drive slots, means shiftably constraining said cam follower members on said platform for movement of said connection elements in substantially mutually perpendicular directions, a first of said cam follower members carrying a cam follower element extending in the same direction as said cam shaft free extremity for engagement with a cam track in the under face of a pattern cam on said cam shaft, a second of said cam follower members being formed with an articulated portion which may be swung alternatively to positions parallel to the cam shaft for exchange of pattern cams or perpendicular to the cam shaft for operation of the attachment, and a cam follower element carried by said articulated portion for engagement with a cam track in the exposed face of a pattern cam on the cam shaft.
 3. A sewing machine work shifting attachment for monogramming as set forth in claim 2 in which said second of said cam follower members includes a plate carrying said connectioN element, cooperating means on said plate and on said supporting platform for constraining said plate to partake only of translatory movement, and a pair of ears struck up from said plate, in which said the articulated portion of said second cam follower includes a lever arm formed with a pair of downturned ears overlapping the ears of the plate, and in which a pivot pin extends between the overlapped ears of said plate and lever arm.
 4. A sewing machine work shifting attachment for monogramming as set forth in claim 3 in which said lever arm is formed with an aperture snuggly accommodating the free extremity of the cam shaft.
 5. A sewing machine work shifting attachment for monogramming as set forth in claim 4 in which said lever arm is formed with a viewing aperture through which a pattern cam in place on the cam shaft is visible accurately to indicate the position of rotation of a pattern cam.
 6. A sewing machine work shifting attachment for monogramming as set forth in claim 2 in which the mechanism operable by said sewing machine for imparting step-by-step turning movement to said cam shaft includes a toothed ratchet wheel fast with respect to the cam shaft, a pawl oscillated by said sewing machine relatively to said ratchet wheel, said ratchet wheel being formed with a segment without teeth at least as large angularly as the angular oscillation of said pawl, and in which manually operable means are provided for at-will turning of said cam shaft. 